The present invention is directed to a method of and apparatus for fabricating a double walled pipe elbow by sliding an inner pipe into an outer pipe, connecting the ends of the pipes together and then bending the pipes into a desired configuration.
Such pipe elbows are used mainly as exhaust gas manifolds in cars with an intermediate space formed between the outer tube and inner tube being relatively small, for instance, approximately 3 millimeters.
To bend a double wall pipe elbow, where prior to the bending step the outer pipe is connected with the inner pipe in the region of their ends by welding, a bending mandril or arbor is inserted into the inner pipe, to prevent crimping or crushing during the bending step and to maintain the original cross-section.
Due to the bending forces developed during the bending step, the outer pipe is deformed so that at least a part of it comes in contact with the inner pipe, whereby the desired effect of the double wall for heating and sound insulation improvement, as compared to a single walled pipe elbow, is not achieved.
It is known from DE-OS 37 12 193 to fill the intermediate space between the inner and outer pipes with a powder for preventing unwanted deformation of the outer pipe during the bending step.
Such a process is both cumbersome and costly, and, because the double walled pipe elbow is fabricated as a mass production article, is considered as particularly disadvantageous.